Ultrafast Coincidence Imaging

The decomposition of energetic molecules plays a vital role in many elementary chemical processes. The Born-Oppenheimer approximation and the assumption of fast intramolecular vibrational-energy redistribution (IVR) remain the cornerstones of our understanding of chemical reactivity. To explain how their breakdown leads to the formation and breaking of chemical bonds requires new theoretical approaches and methods. Photoexcitation in the ultraviolet (UV) is one of the few ways to selectively excite a molecule to a variety of rovibrational states well above its dissociation limit. At these energies, the interplay between IVR and electronically nonadiabatic processes can be studied more easily than at lower energies. The new UV laser pump-probe technique of time-resolved photoelectron-photoion coincidence imaging spectroscopy (TR-PEPICO imaging) can be used to investigate the chemical dynamics that lead to photodissociation and hence shed light on intramolecular dynamics.

Specifically, we use TR-PEPICO imaging to measure correlated time-resolved photoelectron spectra, photoelectron angular distributions, photofragment kinetic-energy spectra, photofragment angular distributions, and vector correlations of the photodissociation products. All these quantities are extremely sensitive to molecular dynamics.